Olfactory bulb
Encyclopedia
The olfactory bulb is a structure of the vertebrate
forebrain involved in olfaction
, the perception of odors.
(bottom) side of the brain. The olfactory bulb is supported and protected by the cribriform plate
of the ethmoid bone
, which in mammals separates it from the olfactory epithelium
, and which is perforated by olfactory nerve
axons. The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb.
The olfactory bulb transmits smell information from the nose to the brain, and is thus necessary for a proper sense of smell. As a neural circuit, the glomerular layer receives direct input from olfactory nerves, made up of the axons from approximately ten million olfactory receptor neuron
s in the olfactory mucosa
, a region of the nasal cavity
. The ends of the axons cluster in spherical structures known as glomeruli
such that each glomerulus receives input primarily from olfactory receptor neurons that express the same olfactory receptor
. Glomeruli are also permeated by dendrites from neurons called mitral cell
s, which in turn output to the olfactory cortex
. Numerous interneuron types exist in the olfactory bulb including periglomerular cells which synapse within and between glomeruli, and granule cells which synapse with mitral cells.
As a neural circuit, the olfactory bulb has one source of sensory input (axons from olfactory receptor neurons of the olfactory epithelium), and one output (mitral cell axons). As a result, it is generally assumed that it functions as a filter
, as opposed to an associative circuit that has many inputs and many outputs. However, the olfactory bulb also receives "top-down" information from such brain areas as the amygdala
, neocortex
, hippocampus
, locus coeruleus, and substantia nigra
.
With this in mind, its potential functions can be placed into four non-exclusive categories:
While all of these functions could theoretically arise from the olfactory bulb's circuit layout, it is unclear which, if any, of these functions are performed exclusively by the olfactory bulb. By analogy to similar parts of the brain such as the retina
, many researchers have focused on how the olfactory bulb filters incoming information from receptor neurons in space, or how it filters incoming information in time. At the core of these proposed filters are the two classes of interneurons; the periglomerular cells, and the granule cells.
The basal dendrites of mitral cells are connected to interneurons known as granule cells, which by some theories produce lateral inhibition
between mitral cells. It is not clear what the functional role of lateral inhibition would be, though it may be involved in boosting the signal-to-noise ratio of odor signals by silencing the basal firing rate of surrounding non-activated neurons. The synapse between mitral and granule cells is of a rare class of synapses that are "dendro-dendritic" which means that both sides of the synapse are dendrites that release neurotransmitter. In this specific case, mitral cells release the excitatory neurotransmitter glutamate, and granule cells release the inhibitory neurotransmitter Gamma-aminobutyric acid
(GABA). As a result of its bi-directionality, the dendro-dendritic synapse can cause mitral cells to inhibit themselves (auto-inhibition), as well as neighboring mitral cells (lateral inhibition).
Olfaction is distinct from the other sensory system
s where peripheral sensory receptor
s have a relay in the diencephalon
. Therefore the olfactory bulb plays this role for the olfactory system
.
, a distinct sensory epithelium from the main olfactory epithelium
that detects pheromone
s, among other chemical stimuli. As in the main olfactory bulb, axonal input to the accessory olfactory bulb forms synapses with mitral cells within glomeruli. However, mitral cells in the accessory olfactory bulb project their axons to targets in the amygdala
and hypothalamus
, where they may influence aggressive and mating behavior. The olfactory bulb also helps humans sense when danger is near by. There is evidence against the presence of a functional accessory olfactory bulb in humans and other higher primates .
Comparing the structure of the olfactory bulb among vertebrate species, such as the leopard frog and the lab mouse, reveals that they all share the same fundamental layout (five layers containing the nuclei of three major cell types; see "Anatomy" for details), despite being dissimilar in shape and size. Of note, a similar structure is shared by the analogous olfactory center in the fruit fly Drosophila melanogaster
, the antennal lobe
. One possibility is that vertebrate olfactory bulb and insect antennal lobe structure may be similar because they contain an optimal solution to a computational problem
experienced by all olfactory systems and thus may have evolved independently in different phyla.
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
forebrain involved in olfaction
Olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
, the perception of odors.
Anatomy
In most vertebrates, the olfactory bulb is the most rostral (forward) part of the brain. In humans, however, the olfactory bulb is on the inferiorInferior
Inferior means of lower station, rank, degree, or grade . It may also refer to:* Inferiority complex* An anatomical term of location* Inferior angle of the scapula, in the human skeleton...
(bottom) side of the brain. The olfactory bulb is supported and protected by the cribriform plate
Cribriform plate
The cribriform plate of the ethmoid bone is received into the ethmoidal notch of the frontal bone and roofs in the nasal cavities....
of the ethmoid bone
Ethmoid bone
The ethmoid bone is a bone in the skull that separates the nasal cavity from the brain. As such, it is located at the roof of the nose, between the two orbits. The cubical bone is lightweight due to a spongy construction. The ethmoid bone is one of the bones that makes up the orbit of the eye...
, which in mammals separates it from the olfactory epithelium
Olfactory epithelium
The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures about 1 square centimetre and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils...
, and which is perforated by olfactory nerve
Olfactory nerve
The olfactory nerve, or cranial nerve I, is the first of twelve cranial nerves. It is instrumental in the sense of smell. Derived from the embryonic nasal placode, the olfactory nerve is capable of regeneration.-Anatomy:...
axons. The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb.
Main olfactory bulb
The main olfactory bulb has a multi-layered cellular architecture. In order from surface to the center the layers are- Glomerular layerGlomerulus (olfaction)The glomerulus is a spherical structure located in the olfactory bulb of the brain where synapses form between the terminals of the olfactory nerve and the dendrites of mitral, periglomerular and tufted cells...
- External plexiform layer
- Mitral cellMitral cellMitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammalian central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli...
layer - Internal plexiform layer
- Granule cell layer
The olfactory bulb transmits smell information from the nose to the brain, and is thus necessary for a proper sense of smell. As a neural circuit, the glomerular layer receives direct input from olfactory nerves, made up of the axons from approximately ten million olfactory receptor neuron
Olfactory receptor neuron
An olfactory receptor neuron , also called an olfactory sensory neuron , is a transduction cell within the olfactory system. J. Rospars, Dendritic integration in olfactory sensory neurons: a steady-state analysis of how the neuron structure and neuron environment influence the coding of odor...
s in the olfactory mucosa
Olfactory mucosa
The olfactory mucosa is located in the upper region of the nasal cavity and is made up of the olfactory epithelium and the underlying lamina propria, connective tissue containing fibroblasts, blood vessels, Bowman's glands and bundles of fine axons from the olfactory neurons.The mucus protects the...
, a region of the nasal cavity
Nasal cavity
The nasal cavity is a large air filled space above and behind the nose in the middle of the face.- Function :The nasal cavity conditions the air to be received by the other areas of the respiratory tract...
. The ends of the axons cluster in spherical structures known as glomeruli
Glomerulus (olfaction)
The glomerulus is a spherical structure located in the olfactory bulb of the brain where synapses form between the terminals of the olfactory nerve and the dendrites of mitral, periglomerular and tufted cells...
such that each glomerulus receives input primarily from olfactory receptor neurons that express the same olfactory receptor
Olfactory receptor
Olfactory receptors expressed in the cell membranes of olfactory receptor neurons are responsible for the detection of odor molecules. Activated olfactory receptors are the initial player in a signal transduction cascade which ultimately produces a nerve impulse which is transmitted to the brain...
. Glomeruli are also permeated by dendrites from neurons called mitral cell
Mitral cell
Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammalian central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli...
s, which in turn output to the olfactory cortex
Piriform cortex
In anatomy of animals, the piriform cortex, or pyriform cortex is a region in the brain.-Anatomy and function:The piriform cortex is part of the rhinencephalon situated in the telencephalon....
. Numerous interneuron types exist in the olfactory bulb including periglomerular cells which synapse within and between glomeruli, and granule cells which synapse with mitral cells.
As a neural circuit, the olfactory bulb has one source of sensory input (axons from olfactory receptor neurons of the olfactory epithelium), and one output (mitral cell axons). As a result, it is generally assumed that it functions as a filter
Filter (signal processing)
In signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal...
, as opposed to an associative circuit that has many inputs and many outputs. However, the olfactory bulb also receives "top-down" information from such brain areas as the amygdala
Amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...
, neocortex
Neocortex
The neocortex , also called the neopallium and isocortex , is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI...
, hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
, locus coeruleus, and substantia nigra
Substantia nigra
The substantia nigra is a brain structure located in the mesencephalon that plays an important role in reward, addiction, and movement. Substantia nigra is Latin for "black substance", as parts of the substantia nigra appear darker than neighboring areas due to high levels of melanin in...
.
With this in mind, its potential functions can be placed into four non-exclusive categories:
- discriminating among odors
- enhancing sensitivity of odor detection
- filtering out many background odors to enhance the transmission of a few select odors
- permitting higher brain areas involved in arousal and attention to modify the detection or the discrimination of odors
While all of these functions could theoretically arise from the olfactory bulb's circuit layout, it is unclear which, if any, of these functions are performed exclusively by the olfactory bulb. By analogy to similar parts of the brain such as the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
, many researchers have focused on how the olfactory bulb filters incoming information from receptor neurons in space, or how it filters incoming information in time. At the core of these proposed filters are the two classes of interneurons; the periglomerular cells, and the granule cells.
The basal dendrites of mitral cells are connected to interneurons known as granule cells, which by some theories produce lateral inhibition
Lateral inhibition
In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition sharpens the spatial profile of excitation in response to a localized stimulus.-Sensory inhibition:...
between mitral cells. It is not clear what the functional role of lateral inhibition would be, though it may be involved in boosting the signal-to-noise ratio of odor signals by silencing the basal firing rate of surrounding non-activated neurons. The synapse between mitral and granule cells is of a rare class of synapses that are "dendro-dendritic" which means that both sides of the synapse are dendrites that release neurotransmitter. In this specific case, mitral cells release the excitatory neurotransmitter glutamate, and granule cells release the inhibitory neurotransmitter Gamma-aminobutyric acid
Gamma-aminobutyric acid
γ-Aminobutyric acid is the chief inhibitory neurotransmitter in the mammalian central nervous system. It plays a role in regulating neuronal excitability throughout the nervous system...
(GABA). As a result of its bi-directionality, the dendro-dendritic synapse can cause mitral cells to inhibit themselves (auto-inhibition), as well as neighboring mitral cells (lateral inhibition).
Olfaction is distinct from the other sensory system
Sensory system
A sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic...
s where peripheral sensory receptor
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
s have a relay in the diencephalon
Diencephalon
The diencephalon is the region of the vertebrate neural tube which gives rise to posterior forebrain structures. In development, the forebrain develops from the prosencephalon, the most anterior vesicle of the neural tube which later forms both the diencephalon and the...
. Therefore the olfactory bulb plays this role for the olfactory system
Olfactory system
The olfactory system is the sensory system used for olfaction, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a main olfactory system and an accessory olfactory system. The main olfactory system detects volatile, airborne substances, while the...
.
Accessory olfactory bulb
The accessory olfactory bulb, which resides on the dorsal-posterior region of the main olfactory bulb, forms a parallel pathway independent from the main olfactory bulb. It is the second processing stage of the accessory olfactory system. It receives axonal input from the vomeronasal organVomeronasal organ
The vomeronasal organ , or Jacobson's organ, is an auxiliary olfactory sense organ that is found in many animals. It was discovered by Frederik Ruysch and later by Ludwig Jacobson in 1813....
, a distinct sensory epithelium from the main olfactory epithelium
Olfactory epithelium
The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures about 1 square centimetre and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils...
that detects pheromone
Pheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...
s, among other chemical stimuli. As in the main olfactory bulb, axonal input to the accessory olfactory bulb forms synapses with mitral cells within glomeruli. However, mitral cells in the accessory olfactory bulb project their axons to targets in the amygdala
Amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...
and hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
, where they may influence aggressive and mating behavior. The olfactory bulb also helps humans sense when danger is near by. There is evidence against the presence of a functional accessory olfactory bulb in humans and other higher primates .
Evolution
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
, the antennal lobe
Antennal lobe
The antennal lobe is the deutocerebral neuropil of insect which receives the input from the olfactory sensory neurons on the antenna. Functionally, it shares some similarities with the olfactory bulb in vertebrates....
. One possibility is that vertebrate olfactory bulb and insect antennal lobe structure may be similar because they contain an optimal solution to a computational problem
Computational problem
In theoretical computer science, a computational problem is a mathematical object representing a collection of questions that computers might want to solve. For example, the problem of factoring...
experienced by all olfactory systems and thus may have evolved independently in different phyla.